To boost the development of future supercomputers and data centers, ideal optical modulators with ultrahigh speed, small footprint, large optical bandwidth, athermal operation, and complementary metal-oxide semiconductor (CMOS) compatibility are needed. Modulators are classified in two operational categories: electrorefractive and electroabsorptive. For the refractive approach, the modulation is typically achieved by varying the plasma dispersion effect and free carrier absorption in silicon to control the real part of material permittivity. However, for a single-pass two-beam interference like Mach-Zehnder's, such a change is typically poor. Hence, a device several hundreds of micrometers long must be employed to manipulate the relative phase of the interfering beams for output power control. This results in a large footprint and a high capacitance, which consequently raises the power consumption. Other refractive modulator designs with multiple-pass single-beam interference, such as resonators, require a large quality factor (>104) or a narrowband modulation (<0.1 nm), which results in a stringent fabrication process. In addition, precise temperature stabilization to keep the device on resonance is needed, causing an increase in the total power consumption.
In contrast, absorptive modulators (such as germanium-based devices) utilize the changes of the imaginary part of the material permittivity by applying an electrical field through the structure, mostly with a reverse bias voltage on a p-i-n-like structure. The electroabsorption effect of germanium has offered a high modulation speed but with a limited optical bandwidth due to finite band gap. As a result, it cannot cover the entire optical communication regime (1525-1565 nm (C band) and 1570-1610 nm (L band)). Furthermore, CMOS-compatible applications require special processes (such as epitaxial growth, wafer bonding, or die bonding), which limits the thermal stability of the final devices.